THE URODELE EYE 601 



resemblances between urodele and anuran eyes are by no means co- 

 incidental, but they represent only inheritances from a common ancestry 

 of great antiquity — probably, a single stegocephalian type. Otherwise, 

 some of the truly remarkable similarities would be hard to explain 

 inasmuch as it is certain that neither the urodeles were ancestral to the 

 anurans, nor rice versa. 



The Eye as a Whole — Accompanying the usual oculorotatory muscles 

 is a retractor bulbi which, as in anurans, may be contracted not only to 

 protect the eyeball but also to use the latter as an aid to swallowing; 

 for, the partition between orbit and mouth cavity is purely membranous. 

 The lacrimal and Harderian glands are about equally prominent. Both 

 are distributed along the lower lid, and are sometimes not discriminable. 

 The lids form at metamorphosis if at all — they are lacking in the 

 permanently aquatic forms; and there is never more than a rudimentary 

 'nictitans'. 



The eyeball is spherical excepting in some of the good-eyed aquatic 

 forms (e.g., Onychodactylus, some newts), in which the cornea is some- 

 what flattened in a fish-like manner. The cornea shows the same layers 

 as that of Rana, and likewise is completed at metamorphosis in those 

 salamanders which thereafter live on land. In the proteid Necturus, the 

 pAmary cornea also fuses with the skin; but since no sulcus then forms 

 around the eyeball, the latter is completely immobilized despite the pres- 

 ence of tiny extra-ocular muscles. This same situation perhaps obtains 

 in some of the other half -transformed, permanently aquatic salamanders. 



The lens being relatively enormous in keeping with the generally 

 secretive habits of the group, the anterior chamber is shallow as com- 

 pared with that of an anuran. The sclera contains the expected hyaline 

 cartilage, but this is subject to great variations. In the Ambystomidae 

 it is a cup like that of the Anura, extending forward at least to the 

 equator and persisting throughout life. But in their presumptive ances- 

 tors, the Hynobiidae, the larval eye contains only a ring of cartilage (c/. 

 teleosts), and this becomes fragmented at metamorphosis. In the sala- 

 mandrids and plethodontids a larval ring disappears at metamorphosis, 

 only bits of cartilage remaining in some individuals of certain species 

 ie.g., Triturus pyrrhogdster) . In the monstrous cryptobranchid Megalo- 

 batrachus maximus, on the other hand, the scleral cartilage is disharmon- 

 iously hypertrophied to a degree unmatched elsewhere in the vertebrates 

 — in a horizontal section of the eyeball, two-thirds of its area is cartilage. 



